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Thread: 200hp Turboprop for RV's & others

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    Registered User Captain_John's Avatar
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    200hp Turboprop for RV's & others

    Ok, everyone has read the latest issue of Sport Aviation (April issue) by now and has seen the Turbo RV-4 on the cover. I hit the ATP Website and am... well, quite intrigued to say the least.

    Could someone please nutshell for me how a turboprop works. Then, after the nutshell overview go into some detail?

    I understand it is an oil burner like the one in the basement, but that is about it.

    In the article it says the exhaust stack is too short and causes aerodynamic back pressure limiting it's performance and raising EGT readings. Why can't they leave the short stack and use a venturi tube to aid in gas extraction from the stack at higher speeds? I am just an idiot with probably an overly simplistic idea, but I am interested in the concept to say the least.

    Anyone?

    CJ
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    Super Moderator orion's Avatar
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    Good morning;

    Interesting, isn't it? However, from a more technical standpoint, I would urge caution.

    Basically, a turbine engine works by sucking in air at the front, compressing it through the use of specially designed compressor section(s), then injecting the high pressure air into a combustor, where it is mixed with atomized fuel. It is then ignited (actually the ignition is continuous) and ejected out the back. The hot gasses, before they exit the engine, are passed through the aft turbine, which is connected by a shaft to the intake turbine and compressor stages. The power extracted from the hot gassees by the turbine wheel at the back is used to drive the front compressors.

    If I recall the design correctly, the Solar turbine uses what's called a centrifugal compressor (as opposed to axial). This actually works quite well and is forgiving of the quality of the air coming in. Some turbine engines are very sensitive to the incoming flow since pressure variations can cause compressor stall. Centrifugal compressors don't really care that much and can operate in avariety of conditions.

    Turbines run at constant rpm. The throttle varies the amount of fuel and thus the turbine temperature. As the fuel is increased and the temperature goes up, so does the engine torque. Torque and rpm make horsepower.

    The engine here is actually a turboshaft design, meaning that the power it generates is usable by hooking on to the engine's output shaft. The output shaft of this type of design is actually a secondary shaft which is connected to the engine's turbine shaft and geared down to about 6,000 rpm. This is of course a high rpm for conventional use and so, for propeller use, you need a reduction drive that slows the rpm down further to a more usable range.

    Since it is a constant rpm engine, it needs to be coupled to a constant speed propeller.

    The Solar turbine has been used successfuly as an auxillary power unit on many aircraft and helicopters. But here is where the concern enters in. The engine was never designed for use as a primary airframe power unit nor is it designed for the type of loads imposed by general aviation flight.

    I know ATP has made modifications to the engine and thus they claim that it now is usable for the general aviation application. Personally, I'd be somewhat skeptical.

    There have been at least six or eight companies trying the same thing in the past fifteen years or so, none of them successfully. There are load and dynamic factors in a turbine that cannot be designed for unless you are intimately familiar with the particular engine in question and know how to design components that work in the environment. This cannot be eyeballed.

    Another concern is that these engines are military surplus. Without extensive inspections, it is virtually impossible to state the condition and quality the engine is in when you buy it. Furthermore, good luck in finding support and parts when the current stocks run out.

    It also worries me to see a company like this selling their product without first spending a responsible amount of time on their own aircraft, flight testing the product. In essence, this makes all the initial customers out there their test pilots.

    Finally, I also tend to get a bit suspicious when I read about rather impressive fuel consumtion figures. If you learn anything about small turbines, it should be that they are exremely inefficient. Some of the published figures for the ATP engines sound way too good to be true. Fuel consumtion must be first measured on a dyno and accurately plotted against the power levels. The same plots must then be generated while the engine is installed on an actual operating airframe.

    Often times the dyno numbers and the installed numbers differ quite significantly. So far, to my knowlege, ATP has not published any accurate and verifiable figures.

    In short, the work they are doing is promising and if their claims become verifiable, the engines could be a possible alternative for light aircraft application. As a potential customer though, I would definitely wait a while longer until the company publishes a responsible technical presentation.

  3. #3
    Registered User Captain_John's Avatar
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    Orion, yes it is QUITE interesting!

    First of all thank you for the very extensive explaination! My question, which I think I know the answer to is this:

    RPM's with respect to returning to earth, below that which the propeller control are done with lowering the speed (temperature) of the engine? I am guessing that 6,000 or so reduced RPM speed at output shaft under full load is thereby reduced to something to te tune of 600 - 1,000 RPM?

    I see the "concerns" you speak of. It is a really BIG BUTT, though!

    Kinda hard to overlook a BUTT like that!

    Well, I have some time to watch this unfold. This is the part of me who is glad the airplane is still a ways off.

    CJ
    RV-7 Fuselage

    http://www.rivetbangers.com

    There's an airshow EVERYDAY at Hangar #5!

  4. #4
    Super Moderator orion's Avatar
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    The 6,000 rpm number is the rotation rate that is commonly used in turboshaft engines and if I recall correctly, it is also the output rpm of the Solar apu.

    The subsequent gearbox, where ATP developed one of their own, then takes the 6,000 rpm and reduces it down to whare a light aircraft prop likes to operate normally, or about 2,700 rpm, give or take a bit.

    Keep in mind that this is the rpm that the engine operates at all the time, from idle to full throttle. The only thing that changes with throttle at the prop is the pitch. Returning to earth, the prop will still turn at the 2,700 rpm rate unless you physically shut the engine down.

  5. #5
    Registered User Captain_John's Avatar
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    Hmmmm, Okaaaay then...

    So this is different than the Super King Air that will run without spinning the airscrew? Seems to me the Beech can spool up the turbine without the propeller spinning?

    This design will always spin the prop at 2,700-ish regardless of power setting?

    CJ
    RV-7 Fuselage

    http://www.rivetbangers.com

    There's an airshow EVERYDAY at Hangar #5!

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    some turboshaft / turboprop engines use a free power turbine, which has another stage of turbine not attached to the compressor at all, but has a powershaft running back through the engine connected to a gearbox. it is entirely possible to have the compressor running without having the free power turbine spinning, which is why you can see some helicopters sitting on the ground with their engines idling but the spinny things on top not spinning.

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    Re: 200hp Turboprop for RV's & others

    My watch says its 2010. It's been about 66 years since the first practical turbine aircraft flew. Still no realistic turboprop engines close to 200hp available to experimental general aviation. Innodyn was a good try 7 years ago, but failed.

    Im building an RV7 with a standard IO360 Lycoming engine. Would prefer turbine power.

    Is God against the idea of using a small turbine engine in general aviation? Come on....this can't be that hard to do.

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    Super Moderator orion's Avatar
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    Re: 200hp Turboprop for RV's & others

    The smaller the turbine the lower the efficiency and thus the higher the fuel burn per power output. Then, given the cost, do you want to pay $300k for 200 hp?
    "To live is to learn; to learn is to live" (author unknown)

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    Re: 200hp Turboprop for RV's & others

    Finally, a good explanation. Thanks.

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    Registered User Dan Thomas's Avatar
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    Re: 200hp Turboprop for RV's & others

    Back in the late '90s an outfit was offering a converted APU with a controllable (not a governed constant-speed) prop for about $20K. It produced 150 hp and burned 18 gallons an hour, as compared to about 9 or 10 gph for a 150 hp piston engine at full throttle. The turbine's advantages include lightness and reliability, but the converted APU idea doesn't have the proven reliability of purpose-built turboprops. All of them have PSRUs and we all know by now the pitfalls of various ideas in that area.

    Turbines spin at terrific speeds. They have to, to compress air to 350 psi or so. And they are made of really expensive metals, to take the heat produced in the combustion section. The turbine blades are made of exotic alloys that don't lose their strength when red hot, and most also have numerous tiny channels and air exit jets on them to keep them cool. The turbine engine is far more complicated than the usual four-colour picture would have you believe, and complexity costs money, as do fancy metals.

    Turbines will never be cheap, and they don't make economical sense below 400 hp or so. The smallest practical units are the 400 hp Allison/Rolls-Royce engine found in many light helicopters. The compressor in that engine spins at 51,600 rpm, and the power output turbine at 35,000. You can imagine the need for strength to withstand the centrifugal forces as well as the need for very finely balanced rotating parts. Bearing quality gets pretty critical. The combustion-turbine section burns enough fuel to generate around 1600 hp, and it's doing that, but as with any turbine engine, about three-quarters of the generated power is used just to keep the compressor turning. Your prop gets what's left.

    Most guys using turboprops are using the real deal, like the Garrett TPE331. That engine is found in many commuter-size airplanes, and generates, depending on model, anywhere from 575 to 1100 hp. Others have used the Pratt & Whitney Canada PT-6, which also comes in a wide range of hp.

    Dan

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    Registered User wsimpso1's Avatar
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    Re: 200hp Turboprop for RV's & others

    All of these engines are Solar APU based. Lousy fuel specifics, single shaft single speed operation - Ugh!

    I am holding out for jet fuel diesels...

    Billski

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    Registered User Toobuilder's Avatar
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    Re: 200hp Turboprop for RV's & others

    I own a Solar T62-32 and have been looking for an excuse to hang it on an airplane someday. However, I'm a trained jet engine technician, and I know it's going to be a risky, expensive proposition that will have little more value than the "wow factor" at a fly in. The current "state of the art" homebuilt use for these engines is likely the conversion offered for the Rotorway Exec helicopters, and just a little searching on their site shows that they spend more time working on them than flying.

    My engine will likely end up on a small boat... Just for the "wow" factor...

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    Registered User n2turbines's Avatar
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    Re: 200hp Turboprop for RV's & others

    Regarding a Turboprop, there is a engine developed in the Czech Republic which we are in the process in importing, it is a split-shaft (two shaft) engine which products 240 SHP and weights 128 lbs. and idles at 52% NGP, both Hartsell and Avia make a prop for it .. this is NOT a vapiorware unit .. if you need more infor email me..
    Last edited by n2turbines; April 1st, 2010 at 02:16 PM.

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    Moderator addaon's Avatar
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    Re: 200hp Turboprop for RV's & others

    It's a pity that Rolls Royce has no (public) intention of doing a turboprop from the RR300. It's a 300 hp turboshaft with quite reasonable fuel consumption for a small turbine, good empty weight... and a price that's just a little insane, of course. Still, I have to think that besides being interesting in a wide range of experimentals, it would be a good fit for a Cessna 206 style workhorse.

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    Registered User Tom Nalevanko's Avatar
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    Re: 200hp Turboprop for RV's & others

    Rolls-Royce Launches RR500 Turboprop Engine for General Aviation Market
    News >> Power Plants >> Announcements
    Released on Tuesday, July 29, 2008

    Rolls-Royce expands small engine range RR500 Turboprop For general aviarion market
    Rolls-Royce today announced the launch of a new turboprop engine, the RR500...

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